In all fish species osmoregulation developed in a way that enabled them to maintain the proper balance of salt and water in the water with chemical composition found in their natural habitat. If osmoregulation is disturbed due to adverse conditions (osmotic stress) or a sudden change in the chemical composition of water (osmotic shock), fish might not be able to physiologically cope with it.
Chronic osmotic stress can eventually result in fish’s death and osmotic shock can cause death very quickly.